It is previously known that cellulose fiber containing materials are sized by ketene dimers or carbamoyl chlorides (e.g., Swedish Pat. Nos. 177,927 or 356,088). The hydrophobic effect is obtained by reaction of the ketene dimer group or the carbamoyl chloride group with the free hydroxyl groups of the cellulose fibers, whereby an organic bond is obtained between the cellulose molecule and the sizing agent. In addition to the cellulose reactive group, the sizing agent also contains a hydrocarbon group with hydrophobic properties, and after completion of the reaction, cellulose fibers with water-repellent properties are obtained.
A suitable process for producing paper with hydrophobic properties consists in adding a dispersion of the sizing agent to the fibers in the stock, the dispersion having such properties that the dispersed particles of the sizing agent are attracted to the cellulose fibers. After removal of water from the fiber suspension on the endless wire, the wet sheet is passed through the press and drier section, whereby the dispersion is broken and the sizing agent gets into contact with the fibers. In the sizing process it is assumed that the sizing agent must first melt, spreading over the fibers and then reacting with the hydroxyl groups. The high speed of paper making machines used today results in about a 30 second stay in the drier section, where the average temperature in the paper sheet is 70.degree. - 75.degree.C. This means that the ketene dimer or the carbamoyl chloride does not get time to react completely with the cellulose fibers in the paper making machine, with the result that the paper leaves the machine in a substantially unsized condition. The hydrophobic effect is developed latter, partly on the tambour, partly during a following storage.
However, it is desirable to obtain a more rapid curing so that the paper will have a high sizing when it leaves the paper making machine. This is of importance when it is desired to change the dosage of additives in paper making in such a way that the hydrophobic properties of the produced paper can be evaluated immediately after leaving the machine. Moreover, storage space can be considerably reduced, which space is at present necessary for the above-mentioned aftercuring to take place. In the cases when it is desired in paper making to coat the paper in line, it is likewise necessary that a certain degree of sizing has already been developed in the machine.
It has been suggested in British Pat. No. 1,172,898 to accelerate the curing time of ketene dimers by adding minor amounts of a hydrophobic acyl compound such as fatty acids and derivatives thereof. These compounds also act as extenders for the ketene dimer. However, using stearic acid in combination with ketene dimers requires a relatively high temperature (approximately 100.degree.C) to accelerate the curing, which temperature cannot be obtained in a conventional drier section of a paper making machine.